1. Field of the Invention
The present invention generally relates to an electrophoretic dispersion solution, an image display medium using the electrophoretic dispersion solution, and an image display device using the electrophoretic dispersion solution.
2. Description of the Related Art
Conventionally, CRTs (Cathode Ray Tubes) and liquid crystal displays have been employed for image display devices which display text and images such as static images and moving images. These devices can instantly display and rewrite digital data; however, they are not suitable to be used at all times as portable devices. Moreover, there are many disadvantages in that eyes become fatigued due to a long-time operation and images cannot be displayed when the power is off. Meanwhile, when text and static images are distributed or stored as documents and the like, they are recorded on a paper medium by a printer. This paper medium is widely used as what is called a hard copy. The text and static image on the hard copy can be more easily read with less fatigue in a free posture. Further, the lightweight hard copy provides superior portability. However, when a hard copy discarded after use is recycled, the recycling requires considerable labor and cost. Therefore, there remains a problem in view of resource savings.
A paper-like rewritable display medium with advantages of both the display device and the hard copy has been in high demand. Display mediums employing high molecular dispersive type liquid crystals, bistable cholesteric liquid crystals, electrochromic elements, electrophoretic elements, or the like have been attracting attention as a reflective type display medium capable of bright display. Among these, a display medium employing electrophoretic elements is superior in display quality and reduced power consumption of a display operation. For example, Patent Documents 1, 2, and the like disclose such display media. A display medium employing the electrophoretic elements includes a dispersion solution filling in between a pair of transparent electrodes. The dispersion solution includes a colored dispersion medium in which plural electrophoretic particles having a different color than the dispersion medium are dispersed. In this case, the electrophoretic particles have charges on the surfaces in the dispersion medium. When a voltage of an opposite polarity (direction) to the charge of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles are attracted to the transparent electrode to which the voltage is applied. As a result, the color of the electrophoretic particles can be observed from the transparent electrode side. When a voltage with the same polarity (direction) as the charge of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles move toward transparent electrode opposite to the one to which the same potential is applied. As a result, the color of the dispersion medium is observed at the transparent electrode side to which the voltage is applied. By utilizing the changes of colors responsive to the voltage change, images can be displayed.
As described above, the paper-like display medium is required to have bistability in addition to viewability and portability. According to Patent Document 3, by using polyisobutylene with a viscosity-average molecular weight of about 4000000 through 1200000, which is dissolved or dispersed in a nonpolar solvent, aggregation is realized by what is called a depletion effect, and the bistability is enhanced.
[Patent Document 1] Japanese Patent No. 2612472
[Patent Document 2] Japanese Patent Application Publication No. 5-173194
[Patent Document 3] U.S. Patent Application Publication No. 2002-0180687
As disclosed in Patent Document 3, when a polymer is dissolved in a dispersion medium to enhance the bistability, viscosity of the dispersion medium is increased. According to the Huckel formula (formula 1 below), electrophoretic mobility and viscosity of a dispersion medium are inversely proportional to each other. That is, when the viscosity of the dispersion medium is increased, the electrophoretic mobility is decreased.
                    μ        =                              2            ⁢                          ɛ              0                        ⁢                          ɛ              r                        ⁢            ζ                                3            ⁢            η                                              [                  Formula          ⁢                                          ⁢          1                ]            
(Here, μ indicates electrophoretic mobility, ∈o denotes a dielectric constant in vacuum, ∈r denotes a relative dielectric constant of the dispersion medium, ζ indicates a zeta potential of the electrophoretic particles, and η indicates a viscosity of the dispersion medium.)
When a polymer is dissolved in a dispersion medium, the viscosity of the dispersion medium is increased and bistability is enhanced. On the other hand, the electrophoretic mobility is decreased, which leads to a delay of an operation, an increase of a driving voltage, and the like of the electrophoretic display medium. That is, in the case where the Huckel formula is established, it becomes very difficult to improve the bistability without increasing the viscosity of the dispersion medium.
It is an object of at least one embodiment of the present invention to provide an electrophoretic dispersion solution having bistability whereby stable electrophoretic mobility and a stable static state can be realized, and to provide an image display medium and an image display device using this electrophoretic dispersion solution.
To realize an electrophoretic dispersion solution having a stable electrophoretic mobility, electrophoretic particles are required to be favorably dispersed. To obtain a favorable dispersed state in a particle dispersed system, a VAN-DER-Waals force (attraction force) and an electrostatic repulsive force (repulsive force), which act between the particles, are required to be controlled, according to DLVO (Derjaguin and Landau, Verwey and Overbeek) logic. In particular, it is generally known that non-DLVO mutual effects strongly act in a nonpolar solvent system. Among the effects, a component related to a polymer repulsive force (repulsive force) is known to be particularly superior. That is, a stable electrophoretic state of the electrophoretic particles in the nonpolar solvent in the particle dispersed system is realized by covering or wrapping (incorporating) surfaces of the electrophoretic particles with a substance with an affinity for the dispersion medium, and forming polymers with an appropriate density and polymerization on the surfaces of the electrophoretic particles.
Moreover, to realize a stable static state, the electrophoretic particles are required to be aggregated. Among plural kinds of electrophoretic particles, one kind with the smallest extinction coefficient in a visible region has little effect on an optical change of image display. Therefore, such a kind of electrophoretic particles does not have to be aggregated to achieve a stable static state.
When an electrophoretic dispersion solution includes n kinds of electrophoretic particles, at least (n−1) kinds of electrophoretic particles are required to be aggregated. The electrophoretic particles may be aggregated between the same kinds of particles or aggregated by becoming attached to a wall surface included as a component of the electrophoretic medium. To obtain a stable aggregated state of the electrophoretic particles, an opposite process to that for forming the dispersed state is to be performed. By a process to remove the polymers on the surfaces of the particles, an attraction force becomes too strong and makes it difficult to form the dispersed state of the electrophoretic particles. Thus, a stable electrophoretic state cannot be obtained.
To obtain an aggregated state of the electrophoretic particles by taking the bistability in consideration, it is effective to form, on surfaces of the electrophoretic particles, polymers including a monomer having a charged group to control a charge property of the electrophoretic particles. Further, in the case where there is a strong mutual effect between different kinds of electrophoretic particles in the electrophoretic dispersion solution including plural kinds of electrophoretic particles, it becomes difficult to cause independent electrophoresis of each of the different kinds of electrophoretic particles. Thus, it becomes difficult for the electrophoretic particles to be eccentrically present in the electrophoretic medium. That is, independent electrophoretic states and independent aggregated states of the different kinds of particles cannot be obtained. For example, one kind of electrophoretic particles disturbs the aggregated state of the other kind of electrophoretic particles. In this case, a stable bistability cannot be obtained. To avoid such a situation, the surfaces of the electrophoretic particles are required to be modified or covered with (incorporated) a substance that is designed to have an appropriate mutual effect between the different kinds of particles and between the particles and the wall surface.
As a result of keen examination of an optimal electrophoretic dispersion solution based on the above description, the present inventors selected a monomer having a charged group and a monomer having a siloxane structure expressed by general formula 1 below as a substance having an affinity for a nonpolar solvent. The inventors found an electrophoretic solution including electrophoretic particles of which surfaces are modified or wrapped with a copolymer including at least these monomers and electrophoretic particles of which surfaces are modified or wrapped with a polymer including at least a monomer expressed by general formula 2 below as a monomer with an affinity for the nonpolar solvent.

In general formula 1, R denotes a hydrogen atom or a methyl group, R′ denotes a hydrogen atom or an alkyl group with a carbon number of 1 through 4, n denotes a natural number, and x denotes an integer of 1 through 3.

In general formula 2, R denotes a hydrogen atom or a methyl group, and R″ denotes an alkyl group with a carbon number of 4 or larger. In particular, the monomer having the charged group is preferably a monomer expressed by general formula 3 or 4.

In general formula 3, R, R1, and R2 independently denote a hydrogen atom or a methyl group, and x denotes an integer of 1 through 3.

In general formula 4, R denotes a hydrogen atom or a methyl group.
By using the above-described electrophoretic solution, bistability can be obtained, whereby a stable electrophoretic state and a stable static state can be realized. When the electrophoretic solution provided in the present invention is used for an image display medium that utilizes an electrophoretic phenomenon, images can be easily displayed in response to a voltage and superior memory can be provided.